How can a linux disk be full, but files total to significanly less?

"Files can be hidden by a mount point.

(A scheduled program (backup) must have run while while I had the secondary volume offline for maintenance, and gleefully recreated it's directory structure rather than failing.)"

"You might have programs running with old unlinked files open. Those files are still stored on disk until those programs close, but will not be linked from the filesystem tree.

Note that this is different from windows behavior, which will not let you delete such a file."